The principle behind the Mutual Multiplication Explosive Tag was not complicated. It simply activated the Summoning Spell Formula as the Explosive Tags detonated, summoned new Explosive Tags, and detonated them, repeating the process over and over... The reason this technique was Forbidden was that it could easily drag the caster into the explosion, causing them to die as well.
For this technique to have any real power, it required an exaggerated number of Explosive Tags. Once the quantity grew too large, it became difficult to control, and being difficult to control meant the caster was almost guaranteed to die.
Aside from that, compared to other Forbidden Techniques, this one was not difficult to learn, because its principles were laid out plainly... The reason it was not commonly seen was that the cost of using it was extremely high.
Taking Kakashi's use of the technique this time as an example, the Explosive Tags alone cost sixty to seventy million ryo, or roughly six million individual tags... Kakashi had probably poured in all his savings since becoming a ninja. It was truly extravagant.
That was why the Mutual Multiplication Explosive Tag never became popular in the ninja world. The main problem was the financial strength of the ninjas.
The explosion inside the hollow golden sphere continued for more than ten seconds. Just as Yuhara felt that his technique was about to lose control of the explosion's power, it finally stopped.
During those ten-odd seconds, the Sunagakure ninjas had miraculously not come over to attack Yuhara and Kakashi, because they were rather dumbfounded...
The Sunagakure ninjas had seen with their own eyes as their Kazekage was stuffed into a sphere.
After the explosion stopped, Yuhara quietly opened a crack in the golden sphere. A pungent smell of burning wafted out, and then he